I'm going to add my theory for drop eye. I think it is caused by the lighting. Since they are top dwellers, having the light on for too long can be hurtful to their eyes. They start to look down as a result - just imagine staring at the sun for long hours. They adapt to their surroundings due to necessity and develop DE.
Now this is just my theory. Could be other reasons as well. Just wanted to add my 2 cents.
Now this is just my theory. Could be other reasons as well. Just wanted to add my 2 cents.